The invention was developed in connection with a research program dedicated to increasing the proportion of hot coke particles, present in a fluid coking reactor, which are contacted by atomized oil or bitumen droplets injected into the fluid bed. The invention will be described in that particular context. However it is contemplated that the invention will be useful in other applications (such as fluid catalyst cracking, steam stripping, particle coating and the like) where it is desired to enhance contact between injected atomized liquid droplets and gas-fluidized particles.
The words ‘oil’ and ‘bitumen’ are used interchangeably in this specification. Bitumen is a specie of oil.
A fluid coker at any particular moment typically may contain a column or fluid bed of about 700 tons of hot coke particles passing therethrough. Steam is injected at the base of the reactor, to maintain the hot coke particles in a fluidized state.
Bitumen or oil is injected into the bed in the form of sprays or jets of fine droplets carried by a carrier gas, such as steam. These are very fast-moving jets of very fine droplets. In U.S. Pat. No. 6,003,789, the present assignees disclosed a steam/bitumen pre-mixer and atomizing nozzle which is capable of producing jets comprising droplets of bitumen having a size in the order of 300 microns, carried in steam and moving at a velocity in the order of 300 fsp. The nozzle is mounted to the side wall of the fluid coker, so that it extends through the wall into the contained fluid bed.
In conformance with conventional industry belief, we initially assumed that fine liquid droplets, delivered in a jet produced by such a pre-mixer and nozzle assembly, would be well mixed with coke particles present in a turbulent fluidized bed. It was assumed that individual droplets would contact and adhere to individual hot coke particles and heat transfer would very quickly convert the oil to gas-make and coke.
However, it was noted that agglomerates of oil-wet particles were being formed. They would drop within the reactor chamber and foul the reactor internals at the base of the chamber. This had been a long standing problem associated with fluid coking operations. It became apparent that the high velocity jet of minute oil droplets, supplied by the aforesaid pre-mixer and nozzle assembly, did not eliminate the problem.
These facts suggested to us that some hot coke particles were being coated with too thick a coating of oil, creating a mass transfer limitation. The oil on the particle would fail to sufficiently rapidly convert to hydrocarbon vapor and coke. The wet particles would contact and adhere together to produce the relatively heavy agglomerates, which would fall down through the bed.
We questioned whether the oil droplets were being well mixed with a sufficiently large number of hot coke particles. Our research therefore turned toward investigating the nature of mixing that was actually involved.
Our experimental work indicated:                that there is a primary dispersion zone immediately adjacent the nozzle outlet, wherein the entering jet penetrates into the fluid bed and, due to its momentum, vigorously mixes with a small volume of the bed, creating the initial section of a “plume”. Contact between some droplets and particles ensues in this initial plume section. The plume, now comprised of oil-wet particles and droplets in a matrix of carrier and fluidizing gas, extends out into the main portion of the fluid bed, which we refer to as the secondary dispersion zone. The residence time in the primary dispersion zone is very short—in the order of milliseconds. The residence time in the secondary dispersion zone is much greater, perhaps in the order of several minutes. The oil coatings on the hot coke particles convert to volatized liquid product, gas-make and coke over time, primarily in the secondary dispersion zone;        that oil-wet agglomerates appear at the end of the plume and then tend to drop to the bottom of the reactor;        that the mixing of oil droplets with hot coke particles in the secondary dispersion zone is relatively ineffective. We believe that only about 20–30% of the coke particles in the reactor are contacted by the liquid feed; and        that the jet issuing from the nozzle outlet is compressed significantly by the fluid bed. If the jet is discharged into open air, it will produce a plume perhaps 30 feet in length. However the plume produced in the fluid bed is short and somewhat L-shaped, as illustrated in FIG. 1. The plume might only have a length in the order of 3–4 feet. Otherwise stated, the fluid bed affects the plume by collapsing it.        
These observations led to the conclusion that it would be desirable to increase the proportion of hot coke particles that experience vigorous mixing and exposure to oil droplets in the primary dispersion zone. The present invention is dedicated to that end.